In the mammalian central nervous system (CNS), including man, there are two types of benzodiazepine (BZD) recognition sites, each thought to be associated with a specific protein. The "central" BZD recognition site is associated with the .alpha.- subunit of GABA.sub.A receptors and participates in the gating of chloride (Cl) channels operated by GABA. The "mitochondrial" BZD recognition site (MBR) is associated with a protein in the outer mitochondrial membrane that participates in the regulation of cholesterol access to the mitochondrial inner membrane, where a specific type of cytochrome P.sub.450 catalyzes the side cleavage of cholesterol to form pregnenolone. Both BZD recognition sites are widely represented in the brain. The central type BZD recognition sites are present in neurons whereas the MBRs are abundant in astroglial cells.
Stimulation of MBR by appropriate ligands (e.g. 4-Cl-Diazepam and Alpidem) facilitates cholesterol influx into mitochondria and the formation of pregnenolone, the parent molecule of endogenous steroids. The BZD-induced activation of mitochondrial steroidogenesis occurs in glial cells, which are rich in MBR and produce steroids that can regulate the GABA.sub.A -operated Cl channel. These steroids act on recognition sites located in the transmembrane region of GABA.sub.A receptors and affect the channel gating process. Some pregnenolone derivatives, e.g. 3-.alpha.-hydroxy-5.alpha.-pregnan-20-one(3-.alpha.OH-DHP) and 3.alpha.,21-dihydroxy-5.alpha.-pregnan-20-one(THDOC) positively modulate, and others (pregnenolone sulphate) negatively modulate GABA action. Because these steroids appear to occur naturally in the brain, they are termed neurosteroids.
Research on BZD ligands by the present inventors, targeted to BZD recognition sites located on mitochondria of glial cells, has been carried out in order to provide drugs that can be used to modify neurosteroid production and to rectify neuropsychiatric abnormalities (pathological anxiety, panic, depression etc.) that have as an etiopathogenesis, an alteration in GABAergic function.
It is known that 4-Cl-Diazepam (a high affinity ligand for the MBR that also binds to the GABA.sub.A receptor) in small doses causes a proconflict response (i.e., anxiety) in rats that is partially resistant to Flumazenil, a specific antagonist of BZD at the GABA.sub.A receptors but which is sensitive to PK-11195, a partial agonist at the MBR. On the other hand, Alpidem (an imidazopyridine that has high affinity for the MBR and BZD recognition sites on GABA.sub.A receptors) causes a dose-related anticonflict (anxiolytic) action that is reduced by both PK-11195 and Flumazenil.
Thus, it is difficult to ascribe a behavioral effect on the MBR or modulation of GABA receptors by Alpidem and 4-Cl-Diazepam.
The compounds of the present invention modulate the MBR receptor, and their anxiolytic action can be related exclusively to the binding of MBR since they fail to bind to the BZD receptor site located on GABA receptors or to binding sites for other known putative neurotransmitters.